This invention relates generally to nuclear reactors and more particularly to a nuclear reactor utilizing rotating plugs and a split positive top core holddown structure.
A nuclear reactor produces heat by fissioning of nuclear materials which are fabricated into fuel elements and assembled within a nuclear core situated in a pressure vessel. In commercial nuclear reactors, the heat produced thereby is used to generate electricity. Such nuclear reactors typically comprises one or more primary flow and heat transfer loops, and a corresponding number of secondary flow and heat transfer loops to which conventional steam turbines and electrical generators are coupled. A typical energy conversion process for commercial nuclear reactors, therefore, involves transfer of heat from a nuclear core to the primary coolant flow system, to a secondary coolant flow system and finally into steam from which electricity is generated.
In a liquid cooled nuclear reactor, such as a liquid metal cooled breeder reactor, a reactor coolant, such as liquid sodium, is circulated through the primary coolant flow system. A typical primary coolant flow system comprises a nuclear core within a reactor vessel, a heat exchanger, and a circulating pump. In nuclear reactors having more than one primary flow loop within the primary system, the nuclear core and the reactor pressure vessel are common to each of the primary loops.
The heat generated by the nuclear core is removed by the reactor coolant which flows into the reactor vessel and through the reactor core. The heated reactor coolant then exits from the reactor vessel and flows through the heat exchangers which transfer the heat to secondary flow systems associated therewith. The cooled reactor coolant exits from the heat exchangers and flows to a circulating pump which again circulates the coolant to the pressure vessel, repeating the described flow cycle.
The nuclear reactor pressure vessel is generally sealed at its top by a cover known generally as a closure head. In fast-neutron energy reactors, such as a liquid metal cooled breeder reactors, it is customary for the closure head to include one or more rotatable members known as plugs. By suitable rotation of these plugs, it is possible for the instrumentation, control, and handling equipment located on these plugs to be positioned above all desired locations in the nuclear core. In this manner, it is possible to provide under-the-plug refueling; that is, refueling of the nuclear core occurs while the closure head is maintained in its location atop the pressure vessel and core.
In liquid metal cooled breeder reactors, it is customary to provide a positive top core holddown to help maintain the fuel elements in their position during reactor operations, or in the unlikely event of a core disruptive accident. This core holddown structure, and the upper internals of which it is an integral part, also function to guide, protect, and maintain alignment for the various control mechanisms and instrumentations.
The core holddown structure must be positioned on top of the nuclear core during normal reactor operations. However, during special operations, such as rotation of the plug for refueling, the core holddown structure must be raised off its position on top of the nuclear core before the rotation of the plugs can occur. Additionally, during refueling operations, the core holddown structure must permit accessibility of the nuclear core by the fuel handling equipment.
The prior art attempted to solve this problem by attaching the core holddown structure, and the remainder of the upper internals, to a small rotating plug. During normal reactor operations, this core holddown structure was seated on top of, and completely covered, the nuclear core. For refueling, this small plug and attached core holddown structure was mechanically raised, and rotated out of its position atop of the nuclear core by the rotation of the large plug. Refueling then occurs by means of a grapple, or an expansible arm, which was located underneath the large rotating plug. The problem associated with this method was that, in order to free the top of the nuclear core, the core holddown structure had to be rotated out from atop the nuclear core. This meant that a large free space was necessary around the core for the displacement of the core holddown structure during refueling. This free space had to occur within the reactor pressure vessel, and necessitated the building of a nuclear reactor pressure vessel having a diameter much larger than that of the nuclear core.
Another method utilized in the prior art, as exemplified by U.S. Pat. No. 3,773,616, was to attach the core holddown structure to the small rotatable plug eccentric to the axis of the small plug. In this position, the core holddown structure could be rotated away from its position atop the nuclear core by the rotation of the small plug. Although this method reduced somewhat the size requirements of the pressure vessel, it nevertheless did require additional space within the pressure vessel for the movement of the core holddown structure.
Both of the aforementioned methods required the use of special equipment whose only function was the vertical movement of the core holddown structure off its position atop the nuclear core. This added unwanted complexity to an already complex system.
U.S. Pat. No. 3,862,001 discloses a nuclear reactor having a portion of the upper internals and a portion of a core cover attached to each of two rotatable plugs. This system reduced the size requirements of the pressure vessel to that substantially the same as the size of the core. The system did not, however, provide positive top core holddown, and accordingly did not discuss means for vertically moving the core cover.